Thin section fracture-label was used to determine the distribution of wheat germ agglutinin (WGA) binding sites in intracellular membranes of secretory and non-secretory rat tissues as well as in human leukocytes. In all cases, analysis of the distribution of WGA led to the definition of two endomembrane compartments: one, characterized by absence of the label, includes the membranes of mitochondria and peroxisomes as well as those of the endoplasmic reticulum and nuclear envelope; the other, strongly labeled, comprises the membrane of lysosomes, phagocytic vacuoles, and secretory granules, as well as the plasma membrane. The Golgi apparatus was weekly labelled in all studied tissues. This appears to reflect the short lived presence of fully glycosylated membrane proteins in this organelle.